1. Field of the Invention
The invention is in the field of coherent optical detection techniques. It relates to an optical hybrid and a coherent optical receiver, in which the optical hybrid is used.
2. Prior Art
Coherent optical detection making use of phase diversity enables baseband detection without having to use "phase-locked-loop" (PLL) as it is certainly the case with homodyne detection. As it is thus possible to make efficient use of the electrical and optical bandwidth, phase diversity is therefore very suitable for multichannel coherent optical communication with high bit rates (.gtoreq.1 Gbit/s). In addition, considerations regarding noise suppression, such as thermal noise, make it attractive to use phase diversity at high bit rates.
In order to recover the amplitude and the phase of a coherent optical signal, use is made, in the case of phase diversity of optical hybrids. An optical hybrid is an m.times.n multi-port having m signal inputs and n signal outputs, in which m,n.gtoreq.2, which optical hybrid, at two or more of the outputs, emits coherent products of signals presented at two or more of the inputs, which products have mutually well-defined phase differences. There are two types of such optical hybrids used for phase-diversity. Hybrids of the first type, called the polarisation type, generally achieve the desired phase differences between output signals having different polarisations, this being in contrast to those of the second type, called the coupling type, in which the phase differences between output signals having the same polarisation are achieved. Several versions are known of the polarisation type.
A first version of the polarisation type is a 2.times.2 port based on the combination of a power coupler and a polarisation splitter. In this case, a phase shift of 90.degree. is induced at the input of the polarisation splitter, between the split components, emerging from the splitter, of a mixed signal which comes from the coupler and which consists of a received signal and the signal coming from a local oscillator. Such a first version of a 2.times.2 port is disclosed, for example, by reference [1 ]. A second version, disclosed by reference [2], is a 2.times.2 port based on a power coupler;splitter having polarisation controls in the inputs and linear polarisers, set mutually orthogonally, in the outputs. In a third version, as disclosed by reference [3], setting of the 90.degree. phase difference takes place by first separately polarising the input signals elliptically. The elliptically polarised signals are then combined and split in a power-splitting coupler, and are finally split out, with the aid of two polarisation-splitting couplers, into four signals having relative phases of 0.degree. , 90.degree., 180.degree. and 270.degree..
The sensitivity of all these known versions of polarisation-type multiports is the same if all the signals on the output side of such a multiport are used for detection and thermal noise is supressed as much as possible in the process. The use of more than two detectors in this case, however, makes a receiver complex and, moreover, expensive. If, moreover, balanced detection is used, this results in a decreasing bandwidth of the receiver and an increase in thermal noise. It is therefore desirable not to have to use more than two detectors for the detection. For a two-detector receiver, however, the known polarisation-type optical hybrids are not optimal. In the known polarisation-type m.times.n ports having a 90.degree. phase difference, the throughput is always .ltoreq.25%, that is to say that never more than 25% of the signal power at the input is extracted into each of the outputs used. Since optical hybrids which can be used for phase diversity can also be used for `image-rejection` heterodyne receivers, as disclosed, for example, by reference [4], the same restriction applies to receivers of this type with respect to the throughput.